This invention relates to telecommunication systems and more particularly to methods and apparatus for improving signal-to-interference estimation in radio systems, such as a wideband code division multiple access (WCDMA) system in which dedicated channels may use secondary scrambling codes.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and WCDMA telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focuses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques. Two different codes are used for separating base stations and physical channels in the downlink (base-to-terminal) direction. Scrambling codes are pseudo-noise (pn) sequences that are mainly used for separating the base stations or cells from each other. Channelization codes are orthogonal sequences that are used for separating different physical channels (terminals or users) in each cell or under each scrambling code. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a fast transmit power control mechanism, in which the terminal estimates the signal-to-interference ratio (SIR) for its dedicated physical channel (DPCH), compares the estimated SIR against a reference value, and informs the base station to adjust the base station's transmitted DPCH power to an appropriate level. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a communication system such as a WCDMA system that includes a base station (BS) 100 handling connections with four mobile stations (MSs) 1, 2, 3, 4 that each use downlink (i.e., base-to-mobile or forward) and uplink (i.e., mobile-to-base or reverse) channels. In the downlink, BS 100 transmits to each mobile at a respective power level, and the signals transmitted by BS 100 are spread using orthogonal code words. In the uplink, MS 1-MS 4 transmit to BS 100 at respective power levels. Although not shown, BS 100 also communicates with a radio network controller (RNC), which in turn communicates with a public switched telephone network (PSTN).
The signals transmitted in the exemplary WCDMA system depicted in FIG. 1 can be formed as follows. An information data stream to be transmitted is first multiplied with a channelization code and then the result is multiplied with a scrambling code. The multiplications are usually carried out by exclusive-OR operations, and the information data stream and the scrambling code can have the same or different bit rates. Each information data stream or channel is allocated a unique channelization code, and a plurality of coded information signals simultaneously modulate a radio-frequency carrier signal.
From a mathematical point of view, the transmitted signal tk in the downlink (before D/A conversion and the front-end transmitter) is given by:
                              t          k                =                              ∑                          i              =              0                        M                    ⁢                                                    E                c                i                                      ⁢                          s              k                        ⁢                          c              k              i                        ⁢                          u              i                                                          Eq        .                                  ⁢        1            where M is the number of physical channels transmitted by the base station,
      E    c    i  is the energy per chip on channel i, sk is the scrambling code (QPSK), cki is the channelization code for channel i (BPSK), and ui is the transmitted symbol on channel i. The signal given by Eq. 1 is up-converted to radio frequency, transmitted through the radio channel by the modulated carrier signal, and received and down-converted to a baseband signal in the front-end of a receiver.
At a mobile station or other receiver, the modulated carrier signal is processed to produce an estimate of the original information data stream intended for the receiver. This process is known as demodulation. The composite received baseband spread signal is commonly provided to a rake processor that includes a number of “fingers”, or de-spreaders, that are each assigned to respective ones of selected components, such as multipath echoes or images, in the received signal. Each finger combines a received component with the scrambling sequence and the channelization code so as to de-spread the received composite signal. The rake processor typically de-spreads both sent information data and pilot or training symbols that are included in the composite signal. Various aspects of rake receivers are described in G. Turin, “Introduction to Spread-Spectrum Antimultipath Techniques and Their Application to Urban Digital Radio”, Proc. IEEE, vol. 68, pp. 328-353 (March 1980); U.S. Pat. No. 5,305,349 to Dent for “Quantized Coherent Rake Receiver”; U.S. Patent Application Publication No. 2001/0028677 by Wang et al. for “Apparatus and Methods for Finger Delay Selection in Rake Receivers”; and U.S. patent applications Ser. No. 09/165,647 filed on Oct. 2, 1998, by G. Bottomley for “Method and Apparatus for Interference Cancellation in a Rake Receiver” and Ser. No. 09/344,898 filed on Jun. 25, 1999, by Wang et al. for “Multi-Stage Rake Combining Methods and Apparatus”.
Considering one rake finger, the received signal rk is given by:
                              r          k                =                              h            ⁢                                                  ⁢                                          ∑                                  i                  =                  0                                M                            ⁢                                                                    E                    c                    i                                                  ⁢                                  s                  k                                ⁢                                  c                  k                  i                                ⁢                                  u                  i                                                              +                      e            k                                              Eq        .                                  ⁢        2            where h represents the radio channel and ek is noise. In order to retrieve the information transmitted on a channel, say channel j, the received signal rk described by Eq. 2 is de-spread using the fact that |sk|2=|cki|2=1. The de-spread received signal d is given by:
                                                        d              =                                                1                  SF                                ⁢                                                      ∑                                          k                      =                      1                                        SF                                    ⁢                                                            s                      k                                        *                                          c                      k                      j                                        ⁢                                          r                      k                                                                                                                                              =                                                h                  ⁢                                                            E                      c                      j                                                        ⁢                                      u                    j                                                  +                                                      1                    SF                                    ⁢                                                                          ⁢                                                            ∑                                              k                        =                        1                                            SF                                        ⁢                                                                  s                        k                                            *                                              c                        k                        j                                            ⁢                                              e                        k                                                                                                                                                    Eq        .                                  ⁢        3            since the channelization codes c are mutually orthogonal, i.e.,
                    1        SF            ⁢                        ∑                      k            =            1                    SF                ⁢                              c            j                    ⁢                      c            i                                =                                        0            /            1                    ⁢                                          ⁢          if          ⁢                                          ⁢          j                ≠                  i          /          j                    =      i        ,and where * indicates the complex conjugate and SF is the spreading factor used for channel j. When the channel-specific spreading codes are orthogonal to one another, the received signal can be correlated with a particular channel (user) spreading code such that only the desired signal related to a particular spreading code is enhanced while the other signals for all the other users (channels) that are orthogonal to the wanted signal are not enhanced.
In order to do good power control, the SIR on the DPCH needs to be estimated. The signal power S, i.e., |h|2Ecj in Eq. 3, is usually estimated using the DPCH pilot symbols, i.e., known symbols transmitted on the DPCH. The interference I, however, is typically estimated using pilot symbols transmitted on the Common Pilot Channel (CPICH), i.e., a channel with large signal strength, and then scaling to the DPCH interference. The interference on the CPICH ICPICH is estimated by:
            I      ^        CPICH    =            1              N        -        1              ⁢                  ∑                  k          =          1                N            ⁢                                                            d              k              CPICH                        -                                          h                ^                            ⁢                                                          ⁢                              u                k                CPICH                                                              2            where N is the number of symbols used in the estimation, dkx is the k-th symbol de-spread with respect to the scrambling and channelization codes for the channel x, and ukkx is the k-th transmitted symbol on channel x. The estimated interference on the CPICH ÎCPICH is scaled to the estimated DPCH interference ÎDPCH according to:
                                          I            ^                    DPCH                =                                            SF              CPICH                                      SF              DPCH                                ⁢                                    I              ^                        CPICH                                              Eq        .                                  ⁢        4            where Î is the estimated interference power on the channel identified by the subscript and SFCPICH=256 is the spreading factor of the CPICH. The reason for estimating the interference in this way is that an interference estimate based on the CPICH is better, e.g., less noisy, than an I-estimate based on the DPCH pilot symbols. Methods for estimating S and I in a SIR are well known in the art.
According to the WCDMA standard, a BS may use more than one (the primary) scrambling code, i.e., so-called secondary or alternative scrambling codes. For example, when the channelization code tree is full for the primary scrambling code and there is capacity (i.e., transmitter power) left in the BS, the BS can use a secondary scrambling code for DPCHs For another example, when a terminal (e.g., a mobile station) enters compressed mode that entails reducing the spreading factor in the compressed frames, the terminal can use an alternative scrambling code in the compressed frames in order to avoid reallocation of all codes in the code tree due to the terminal's need for a smaller spreading factor. This is described at 3GPP TS 25.213, sections 5.2.1 and 5.2.2.
For example, orthogonal variable spreading factor (OVSF) codes are used in order to maintain link orthogonality while accommodating different user data rates. The OVSF scheme is a kind of code tree, an example of which is depicted in FIG. 2. This example assumes a binary alphabet and mother code of “1”, although the reader will understand that the concept can be generalized. Each level in the tree is a set of codes that are mutually orthogonal. For example, the third level is the set of four length-4 codes “1111”, “1100”, “1010”, and “1001”, and a CDMA system employing this code tree can thus support four simultaneous orthogonal users using codes of SF=4. Alternatively, the system could support eight simultaneous orthogonal users of SF=8 (the fourth level in the code tree), etc. Since the chip rate in a direct-sequence CDMA system is typically constant, a higher SF generally corresponds to a lower information bit-rate.
It is also desirable to support a plurality of simultaneous orthogonal users having different information bit-rates, i.e., different SF values, and this can be done using a code at a given “branch” in the code tree if and only if no code on the path leading to the root of the tree is also used. This restriction on code selection from the tree preserves orthogonality between the selected codes used in a cell. FIG. 3 depicts three different channelization codes (“10”,“1100”, and “11111111”) used to spread three information streams, which may have arisen from three of the mobile stations depicted in FIG. 1. These three different channelization codes provide three different spreading factors, which correspond to three different user information bit rates. Whichever of these three rates is being detected (de-spread) in a receiver, e.g., a receiver at a base station, the other two will always be orthogonal, assuming that they are transmitted synchronously. For instance, when de-spreading the slowest-symbol-rate signal (“11111111”), the eight chips of each symbol are simply accumulated, and the accumulated contribution from the other two signals during the eight chips will be identically zero (in “+1”/“−1” representation, corresponding to BPSK modulation of the “1”/“0” bits).
In cases in which primary and secondary scrambling codes are used, the transmitted signal tk can be written as:
                              t          k                =                                            ∑                              i                =                0                                            M                p                                      ⁢                                                            E                  c                  i                                            ⁢                              s                k                p                            ⁢                              c                k                i                            ⁢                              u                i                                              +                                    ∑                              j                =                0                                            M                s                                      ⁢                                                            E                  c                  j                                            ⁢                              s                k                s                            ⁢                              c                k                j                            ⁢                              u                j                                                                        Eq        .                                  ⁢        5            where sp and ss are the primary and secondary scrambling codes, respectively, and Mp and Ms are the number of physical channels under the respective scrambling code (typically, Ms<<Mp). The received signal rk at one finger of the rake receiver can then be modeled as:
                              r          k                =                              h            ⁡                          (                                                                    ∑                                          i                      =                      0                                                              M                      p                                                        ⁢                                                                                    E                        c                        i                                                              ⁢                                          s                      k                      p                                        ⁢                                          c                      k                      i                                        ⁢                                          u                      i                                                                      +                                                      ∑                                          j                      =                      0                                                              M                      s                                                        ⁢                                                                                    E                        c                        j                                                              ⁢                                          s                      k                      s                                        ⁢                                          c                      k                      j                                        ⁢                                          u                      j                                                                                  )                                +                      e            k                                              Eq        .                                  ⁢        6            Assuming a desired DPCH uses a secondary scrambling code (channel code 1), we get the following de-spread received signal d:
                                                        d              =                                                1                  SF                                ⁢                                                      ∑                                          k                      =                      1                                        SF                                    ⁢                                                            s                      k                      s                                        *                                          c                      k                      1                                        ⁢                                          r                      k                                                                                                                                              =                                                h                  ⁢                                                            E                      c                      1                                                        ⁢                                      u                    1                                                  +                                                      1                    SF                                    ⁢                                                                          ⁢                                                            ∑                                              k                        =                        1                                            SF                                        ⁢                                                                  s                        k                        s                                            *                                                                        c                          k                          1                                                (                                                                              h                            ⁢                                                                                                                  ⁢                                                                                          ∑                                                                  i                                  =                                  0                                                                                                  M                                  p                                                                                            ⁢                                                                                                                                    E                                    c                                    i                                                                                                  ⁢                                                                  s                                  k                                  p                                                                ⁢                                                                  c                                  k                                  i                                                                ⁢                                                                  u                                  i                                                                                                                                              +                                                      e                            k                                                                          )                                                                                                                                                    Eq        .                                  ⁢        7            
As can be seen from Eq. 7, the channels under the primary scrambling code are transformed to white noise in a channel under a secondary scrambling code since the scrambling codes are pn-sequences and not orthogonal codes. Hence, the interference (noise) n1 affecting the desired code 1 is given by:
                              n          1                =                              1            SF                    ⁢                                    ∑                              k                =                1                            SF                        ⁢                                          s                k                s                            *                                                c                  k                  1                                (                                                      h                    ⁢                                                                                  ⁢                                                                  ∑                                                  i                          =                          0                                                                          M                          p                                                                    ⁢                                                                                                    E                            c                            i                                                                          ⁢                                                  s                          k                          p                                                ⁢                                                  c                          k                          i                                                ⁢                                                  u                          i                                                                                                      +                                      e                    k                                                  )                                                                        Eq        .                                  ⁢        8            and the interference power (noise power) I1 is given by:
                              I          1                =                              1            SF                    ⁢                      (                                                                                                    h                                                        2                                ⁢                                                                  ⁢                                                      ∑                                          i                      =                      0                                                              M                      p                                                        ⁢                                      E                    c                    i                                                              +                              σ                e                2                                      )                                              Eq        .                                  ⁢        9            where σe2 is the variance of the noise ek. Using the I-estimate obtained from the CPICH (which is a channel always sorted under the primary scrambling code, channelization code 0), the interference (noise) n2 is given by:
                              n          2                =                              1            SF                    ⁢                                          ⁢                                    ∑                              k                =                1                            SF                        ⁢                                          s                k                p                            *                                                c                  k                  0                                ⁡                                  (                                                            h                      ⁢                                                                                          ⁢                                                                        ∑                                                      i                            =                            0                                                                                M                            s                                                                          ⁢                                                                                                            E                              c                              i                                                                                ⁢                                                      s                            k                            s                                                    ⁢                                                      c                            k                            i                                                    ⁢                                                      u                            i                                                                                                                +                                          e                      k                                                        )                                                                                        Eq        .                                  ⁢        10            and the interference power (noise power) I2 is given by:
                              I          2                =                                                          h                                      2                    ⁢                      1            SF                    ⁢                      (                                                            ∑                                      i                    =                    0                                                        M                    s                                                  ⁢                                  E                  c                  i                                            +                              σ                e                2                                      )                                              Eq        .                                  ⁢        11            
It is easily seen that Eqs. 8 and 9 are not equivalent to Eqs. 10 and 11. Furthermore, computer simulations indicate that the loss in a power-controlled environment using an I-estimate obtained from a CPICH using one scrambling code while the DPCH uses another scrambling code can be several dB in terms of average needed BS power on the DPCH. Hence, in cases when alternative scrambling codes are used, there is a need for a better way to estimate the interference than the conventional CPICH I-estimate approach.
One simple known solution to this problem is to use the DPCH pilot symbols for both signal power (S) estimation and interference power (I) estimation, but this solution has problems. For example, the I-estimate is noisy because the number of DPCH pilot symbols is small and the DPCH's overall signal power is small since the DPCH is power-controlled. The noisy I-estimate produces a noisy SIR estimate, and since the SIR estimate directly affects the average needed BS DPCH power, erroneously determining the average power due to the noisy SIR estimate can reduce the system capacity.
Hence, there is a need for methods and apparatus capable of better estimating interference power in cases such as those in which alternative scrambling codes are used in WCDMA such that the performance, in terms of accuracy in the SIR estimate, becomes similar to the case in which the DPCH is sorted under the primary scrambling code, i.e., the scrambling code under which the CPICH is sorted.